378 



WHITEHEAD— HIGH VOLTAGE CORONA IX AIR. [April 21. 



is given in Table I. and the results are plotted graphically in Fig. 2. 

 For comparison, points observed by other investigators are also 

 shown. The values are all corrected for temperature, pressure and 

 wave form and give the maximum values of the electric intensity 

 at which the air breaks down under a pressure of 760 mm. of 



TABLE I. 

 Rel.\tion Between Di.a.meter .\nd Critical Surface Intensity. 



mercury, and at temperature 21° C. I am indebted to Professor 

 Alexander Russell for pointing out that these results obey a very 

 simple law. If E be the critical electric intensity in kilovolts per 

 centimeter and d the diameter of the conductor in centimeters, the 

 curve of Fig. 2 obeys closely the ecjuation : 



£ = 32 + 13-4 i/V^- (i) 



The observed values of Table I. are compared with the values cal- 

 culated from the above formula in Table II. The percentage error 

 is also given, and it is seen that with one exception the difference 

 is well within one per cent. The exception refers to an aluminum 

 wire which could not be polished to a clean surface ; a rough sur- 



